A typical fuel pumping unit for an aero-engine comprises a low pressure (LP) pump operable to draw fuel from a fuel tank, and supplying the fuel at boosted pressure to the inlet of a high pressure (HP) pump. The inter-stage flow between LP and HP pumps is typically used to cool engine lubrication oil in a fuel/oil heat exchanger.
Commonly, the LP pump comprises a centrifugal impeller pump whilst the HP pump comprises a positive displacement pump in the form of a twin pinion gear pump. The pumps are generally driven from a shaft on the engine accessory gearbox and therefore have a fixed speed relationship with engine running speed. A rotating seal is typically required to avoid leakage around the input drive shaft to the gear pump.
FIG. 1 shows schematically a conventional twin pinion gear arrangement with first 2 and second 4 pinion gears mounted on parallel first 6 and second 8 gear shafts, the gears meshing at region R.
A positive displacement pump, such as a gear pump, is typically configured to provide sufficient flow at the maximum take-off or the windmill-relight design points of an aero-engine. However, the pump generally only needs to provide a small fraction of this flow under normal operation, with the excess flow being re-circulated.